The invention relates to a flexographic printing press with a back-pressure cylinder and at least one inking unit, which has an exchangeable printing cylinder, which can be placed against the back-pressure cylinder, an engraved ink transfer cylinder, which can be placed against the printing cylinder and a chamber doctor blade, which can be placed against the engraved ink transfer cylinder.
In the case of such a printing press, the printing cylinders and, as a rule, also the engraved ink transfer cylinders of the various inking units can be exchanged, so that the machine can be re-equipped for different printing tasks. In EP-A-0 769 373, a flexographic printing press is described, for which a particularly simple exchange of printing cylinders is made possible owing to the fact that the actual printing cylinder is constructed as a so-called sleeve, which is disposed removably on an axle, which is held in the machine frame, and can be pulled off axially from this axle. U.S. Pat. No. 3,378,902 discloses a system, which permits the sleeve to be clamped hydraulically on the axle. Further examples of printing presses, for which the printing cylinder and the engraved ink transfer cylinder can be exchanged easily, are described in EP-A-0 812 681 and in EP-A-1 090 754. EP-A-1 010 522 discloses a robot, with which the exchange of cylinders can be automated.
In the case of conventional flexographic printing presses, the driving mechanisms for the back-pressure cylinder and the printing cylinder of the individual inking units are coupled mechanically with one another. On the other hand, EP-A-1 000 737 described a flexographic printing press with individual driving mechanisms, with which the rpm of the printing cylinders can be selected independently of the rpm of the back-pressure cylinder.
In the EP-A-0 970 810, a system is described for cleaning the engraved ink transfer cylinder and the chamber doctor blade. In this case, a system of pipelines, which is normally used to supply printing ink to the chamber doctor blade and to discharge excess printing ink from the chamber doctor blade, is employed for flushing the chamber doctor blade with a cleaning liquid. At the same time, the peripheral surface of the engraved ink transfer cylinder can also be cleaned.
Since contamination, which can affect the printing quality, accumulates also on the surface of the back-pressure cylinder during the course of the printing process, it is necessary to wash off the printing cylinder from time to time with a liquid solvent. Until now, this has been done manually in a relatively time-consuming and work-intensive manner.
It is an object of the invention to equip a flexographic printing press with a washing device, which facilitates the washing of the back-pressure cylinder.
Pursuant to the invention, this objective is accomplished by a washing cylinder, which is constructed to be inserted in one of the inking units instead of the printing cylinder and to apply cleaning fluid to the surface of the back-pressure cylinder.
Accordingly, since the washing cylinder takes the place of the printing cylinder in one of the inking units during the washing process, hardly any additional space is required at the periphery of the back-pressure cylinder for accommodating the washing device. By these means, a compact construction of the printing press becomes possible. A further advantage consists therein that the components of the washing device are present anyhow in the printing press. Accordingly, especially the driving mechanism, which is normally provided for the printing cylinder, can also be used as driving mechanism for the washing cylinder and the placement of the washing cylinder in contact with the back-pressure cylinder can also be brought about with the help of the mechanisms, which are otherwise used for the printing cylinder. In preparation for the washing process, the printing cylinder of one of the inking units must be exchanged for the washing cylinder. With the help of the devices, provided for exchanging printing cylinders, as described, for example, in the publications named above, this process can also be simplified or automated.
If the inking unit, into which the washing cylinder is to be installed, has a known device for flushing the chamber doctor blade with cleaning liquid, this chamber doctor blade and the engraved ink transfer cylinder can also be used for moistening the washing cylinder with cleaning liquid, so that additional devices for supplying cleaning liquid to the washing cylinder are not required. So that the washing cylinder is supplied with a sufficient amount of cleaning liquid, it may be appropriate for the washing process to exchange the engraved ink transfer cylinder, normally used for the printing presses, for a special engraved ink transfer cylinder, which is capable of transferring a larger amount of liquid.
The cleaning liquid, which is applied to the surface of the back-pressure cylinder, preferably is removed with the help of a doctor blade, which can be placed against and removed from the periphery of the back-pressure cylinder and is disposed in the vicinity of the lower apex of this cylinder, together with the contaminants, which have been stripped off from the back-pressure cylinder by means of this cleaning liquid. This doctor blade then represents the only component of the washing device, which must be disposed additionally at the periphery of the back-pressure cylinder.
The invention is particularly advantageous in the case of a flexographic printing press with individual driving mechanisms, since in that case the peripheral velocity, selected for the washing cylinder, can be different from that selected for the back-pressure cylinder, so that a wiping motion takes place at the contact area. Alternatively, the washing cylinder can also be driven in the opposite direction to the back-pressure cylinder for this purpose.